Group VIII transition metal compounds are commonly used as homogeneous catalysts for commercially important organic reactions. One such process is hydroformylation, in which an olefin reacts with hydrogen and carbon monoxide in the presence of a Group VIII transition metal compound to give an aldehyde. Hydroformylation of allyl alcohol, for example, gives 4-hydroxybutanal, which is an intermediate in a commercial route to 1,4-butanediol.
A central problem is how best to recover the typically expensive Group VIII transition metal compound from organic mixtures. Conventional approaches to metal recovery include extraction with aqueous solutions, addition of precipitating agents, or a combination of these techniques.
Extraction of Group VIII metals from organic mixtures using aqueous acetic acid (European Patent No. 0 255 389), aqueous amine solutions (U.S. Pat. No. 4,292,196), aqueous alkaline cesium salt solution and crown ether (U.S. Pat. No. 4,363,765), aqueous solutions of ionic organophosphines (U.S. Pat. No. 4,935,550), and amine/HCN mixtures (J. Am. Oil Chemists Soc. 54 (1977) 276) have been described.
Precipitation of the Group VIII metal compound, followed by either extraction or filtration of the precipitate is a second general approach. Examples include precipitation by peroxide treatment of an organic mixture containing the Group VIII metal catalyst (U.S. Pat. No. 3,547,964), reductive treatment with hydrogen/catalyst or a hydride reducing agent (U.S. Pat. No. 4,560,539), precipitation of agglomerated rhodium from neutralized distillation residues (U.S. Pat. Nos. 3,998,622 and 4,135,911), oxidation under basic conditions (U.S. Pat. No. 4,396,551), treatment with an organic sulfur compound to form a precipitate (U.S. Pat. No. 4,413,118), and treatment with a carboxylic acid to precipitate an active catalyst (U.S. Pat. No. 4,950,629).
Ion-exchange methods have been used to recover Group VIII metals from aqueous solutions, as described, for example, in U.S. Pat. Nos. 2,945,743 and 3,567,368. U.S. Pat. No. 3,755,393 teaches a method in which a hydroformylation mixture is passed through a basic ion-exchange resin to recover rhodium. U.S. Pat. No. 4,388,279 teaches a process for recovering Group VIII metals from organic solutions using either a solid absorbent such as calcium sulfate, molecular sieves, or an anionic ion-exchange resin. Acidic ion-exchange resins have not been used to recover Group VIII metals from organic solutions.